Asymmetric Unit

The asymmetric unit (see definition below) is what is contained in the model that crystallographers publish in the Protein Data Bank. It is not to be confused with the biologically functional unit (Biological Unit), which may be the same, smaller, or larger than the asymmetric unit.



Show the:
 * Asymmetric Unit.
 * Biological Unit (obtained from the Probable Quaternary Structure server at the European Bioinformatics Institute). In the case of 2qcb, it is a homotetramer of the asymmetric unit.
 * Unit Cell . In the case of 2qcb, the unit cell contains portions of 16 copies of the asymmetric unit.

The asymmetric unit is the smallest portion of a crystal that, when duplicated and moved by crystal symmetry operations, can produce the unit cell of the crystal. The symmetry operations that are common for biological molecules are rotation, translation, and screw operations (which combine rotation and translation). Mirror symmetry planes occur rarely in crystals of biological molecules due to the presence of chiral centers, e.g. L or D amino acids.

The biological unit is defined elsewhere.

The unit cell is the smallest portion of a crystal that, when duplicated and translated, can generate the entire crystal. You can see the unit cell by clicking on the word Jmol to the lower right of any molecular display in Jmol (which includes most interactive molecular displays in Proteopedia), and then on the menu that appears, Style, then check the Unit Cell checkbox. You can populate the unit cell from Jmol's menu with Symmetry, Reload {1 1 1}.

Websites

 * Diagrams illustrating these concepts are available at the RCSB Protein Data Bank.


 * Glossary of Terms from Crystallography, NMR, and Homology Modeling


 * Asymmetric unit at the Online Dictionary of Crystallography of the International Union of Crystallographers.